I'm quite the diet cola junkie myself, I probably drink 2-3 cans per day as well. I really am not concerned about the aspartame; I have absolutely no faith in the FDA and very little faith in "studies" that change every few years. However, I do think the caffeine and carbonation can be bad; caffeine can give me headaches, so I've switched to caffeine free unless I'm at a restaraunt or something. And I have heard, as several posters mentioned, that carbonation can leach calcium from your bones.

I think it all comes down to moderation. Eating/drinking excessive amounts of nearly anything is bad for you. Personally, I think it's a lot more dangerous to breathe the air of Salt Lake City during a winter inversion (fellow SLC'ers know how disgusting the air here is sometimes) than drinking a can of Diet Coke.

I do appreciate the tip about sparkling water with citrus though, I'll have to give it a try and add some variety to my life :)